1. Field of the Invention
The present invention relates to a shell main body for a muffler in an exhaust system of a combustion engine mounted on a motor vehicle or the like.
2. Description of the Related Art
A shell main body disclosed in Japanese Patent Application Laid-open No. 2002-206422 is conventionally known as a shell main body for a muffler. As shown in a schematic cross-sectional view in FIG. 7, a shell main body 101 of this muffler main body is formed of two layers of an inner shell 102 and an outer shell 103. Each of the inner and outer shells 102 and 103 has a cylindrical shape with a cross section that is not a perfect circle but is defined by small arc portions alternating with large arc portions, similarly to an oval, a racing track-like figure, or the like.
The conventional shell main body for the muffler described above, however, has the following problem. The shell main body 101 is provided at its edge portion with openings, which are closed by end plates by press caulking or welding, respectively, causing minute gaps between them. Consequently, water enters the gaps formed between the inner shell 102 and the outer shell 103 due to capillary phenomenon, and is heated by high-temperature exhaust gas passing through the muffler main body to vaporize, resulting in pressure rise of the vapor. When this pressure rise speed exceeds a discharging speed of the vapor that is discharged from the gaps, vapor pressure in the gaps between the shells 102 and 103 sometimes rises to deform them. Incidentally, water causing such a problem sometimes splashes onto an outer periphery of the shell main body 101 and enters into the gap between the inner shell 102 and the outer shell 103.
Such a problem can be solved if a communicating path, for example a hole, is formed on an upper surface side of the shell main body 101 along its thickness direction so as to allow the gap between the inner shell 102 and the outer shell 103 to communicate with the outside of the outer shell 103, because the vapor entered into the gap can pass through the communicating path of the outer shell 103. However, in a shell main body having a cylindrical shape with a cross section that is not a perfect circle but is defined by small arc portions alternating with large arc portions, similarly to an oval, a racing track, or the like, the inner shell 102 and the outer shell 103 in the right and left small arc portions are in close contact with each other, clogging their right and left side gaps and dividing the gap into an upper and lower gaps in the large arc portions. It is undesirable to provide a lower side of the outer shell 103 with a communicating path connecting the lower gap and the outside of the outer shell 103, because a splash and/or mud is allowed to enter the lower gap through the communicating path and rust the muffler main body. Consequently, vapor entered the lower gap can escape to nowhere, resulting in pressure rise in the lower gap between the inner shell 102 and the outer shell 103. This may possibly deform the inner shell 102 and the outer shell 103.